This invention is related to an electrical alternator, particularly adapted for use in motor vehicle applications including passenger cars and light trucks. These devices are typically mechanically driven using a drive belt wrapped on a pulley connected to the crankshaft of the vehicle's internal combustion engine. The belt drives a pulley on the alternator which rotates an internal rotor assembly to generate alternating current (AC) electrical power. This alternating current electrical power is rectified to direct current (DC) and supplied to the motor vehicle's electrical bus and storage battery.
While alternators have been in use in motor vehicles for many decades, today's demands on motor vehicle design, cost, and performance have placed increasing emphasis on the design of more efficient alternators. Today's motor vehicles feature a dramatic increase in the number of electrical on-board systems and accessories. Such electrical devices include interior and exterior lighting, climate control systems, increasingly sophisticated power train control systems, vehicle stability systems, traction control systems, and anti-lock brake systems. Vehicle audio and telematics systems place further demands on the vehicle's electrical system. Still further challenges in terms of the output capacity of the motor vehicle's electrical alternators will come with the widespread adoption of electrically assisted power steering and electric vehicle braking systems. Compounding these design challenges is the fact that the vehicle's electrical system demands vary widely, irrespective of the engine operating speed which drives the alternator and changes through various driving conditions.
In addition to the challenges of providing high electrical output for the vehicle electrical alternator, further constraints include the desire to minimize the size of the alternator with respect to under hood packaging limitations, and its mass which relates to the vehicle's fuel mileage.
In addition to the need of providing higher electrical output, designers of these devices further strive to provide high efficiency in the conversion of mechanical power delivered by the engine driven belt to electrical power output. Such efficiency translates directly into higher overall thermal efficiency of the motor vehicle and thus into fuel economy gains. And finally, as is the case with all components for mass-produced motor vehicles, cost remains a factor in the competitive offerings of such components to original equipment manufacturers.
It is well known that claw pole style rotors are used almost exclusively in automotive alternators. A standard claw-pole rotor consists of two iron claw pole pieces, an insulating bobbin supporting a wound field coil, a shaft, and a slipring assembly.
The excitation winding consists of a continuous insulated copper wire wrapped around the bobbin. Each claw-pole piece includes a hub portion that is inserted inside the inner diameter of the bobbin. The hub portion of each pole piece has a face area that contacts the opposite hub face area forming a continuous iron magnetic circuit within the bobbin inside diameter.
A knurled shaft is inserted into a bore formed by the assembly of the pole pieces, locking the poles onto the shaft. The pole assembly may contain two or three components. The slipring assembly is also pressed onto a smaller knurled diameter of the shaft. A start and end wire of the field coil are directed across the back of the rear pole piece and up slots in the shaft. The start and end wire are electrically connected to copper shells of the slipring assembly. This allows the electrical brushes mounted onto the machine to pass current through the slipring assembly and field coil while the rotor is rotating with respect to the rest of the machine.
It is also well known that the power density and the efficiency of the alternator can be improved if the percentage of copper fill within the rotor field coil can be increased without saturating the steel magnetic circuit. The more field coil revolutions that can be positioned inside the rotor, the higher the magnetic flux resulting in a higher output for the alternator. Currently, a standard conventional rotor contains an area for the field coil made up by opposing claw-pole pieces. The available area is occupied by the bobbin and field coil. Known designs use approximately 16% of the field coil allowable area for the bobbin. The rest of the area is available for the field coil copper wires.
Generally, bobbins on the market today have a wall thickness of approximately 0.5-0.9 millimeters (mm) thick. Most currently used bobbins are injection molded using nylon 6-6. It is not desirable to manufacture bobbins with a wall thickness of less than 0.5 mm.
Another existing prior art design and process to manufacture alternator bobbins is by stamping the end caps out of a paper or polymer sheet. The bobbin end caps are assembled onto a hub that is a separate piece from the two claw-poles. The hub provides structural rigidity for the inside diameter of the bobbin allowing it to be wound on a wire winder and removed from the winder without the wire tension collapsing the bobbin.
There are two key disadvantages in this design. First, it requires that the hub for the claw-poles be a separate piece from the claw-poles. In this design, the bobbin end caps must be assembled to the hub and fixed in place, often by taping them in place. Then, the field coil wire is wound on the bobbin supported by the hub. Second, stamped end caps are relatively thick to help retain the field wires in position and prevent the winding from flexing the end caps outward once it is removed from the wire winder's support tooling.
There are several reasons why it is not desirable to have a hub that is a separate component from the pole pieces. First, it is more expensive to manufacture since the hub is not made as part of the poles in a fully finished forged part. Rather, the poles are forged separately and the hub is cold headed and machined to final form. Second, it creates an extra interface between the ends of the hub and the pole faces. This reduces the magnetic field conducted through the poles and reduces the power output of the machine. On the other hand, poles that have integrated hubs only have one interface where the pole hub faces meet when assembled. Third, the structural rigidity of each pole is reduced. During high revolution per minute (RPM) operation, the fingers of the poles tend to flex outwardly due to centrifugal force. With the hub removed, the base strength of the poles is reduced leading to a reduction in high RPM capability and increased noise due to vibration.
Further, there are several reasons why it is desirable to decrease the thickness of the stamped end caps. First, the thicker the end caps are, the less field wire fill that is able to fit in the rotor since more space is occupied by the end caps. This reduces the power output of the alternator due to reduced power density and reduces the efficiency of the alternator. Second, heat transfer through the bobbin end caps from the coil to the poles is reduced leading to the field coil operating at a high temperature. This also contributes to reduction in efficiency, power density, and useful life of the alternator.